1. Field of the Invention
This invention relates to a motor-powered tool in which the number of revolutions of the motor is controlled by a switching element such as a thyristor and triac.
2. Description of the Prior Art
FIG. 1 shows a disc grinder 7 as an example of a conventional electrically-powered tool, which comprises a motor housing 1 disposed generally in the mid portion of the disc grinder 7, a gear box 2 connected to the fore-end portion of the motor housing 1, an abrasive wheel 3 rotatably attached to the gear box 2, and a handle 4 attached to the rear-end portion of the motor housing 1. A main switch 5 and a circuit block 6 are incorporated in the handle 4. A motor is shown generally at 8 in the motor housing 1, having an armature shaft 9 and a magnetic ring 11 connected to the armature shaft 9 through a cylindrical body 10. A switching element 12 is provided in the circuit block 6, being electrically controlled by the motor 8 revolution detecting signals produced by the magnetic ring 11 and a magnetism-sensitive element (not shown). The problem associated with this switching element 12 is that its capacitance is so large as to require a fairly large space for cooling itself. Because of this extra space, the external configuration of the circuit block 6 including such a switching element 12 is apt to be large. The longitudinal length of the disc grinder 7, as indicated by L in FIG. 1, is often 50 to 60 mm relative to an outside diameter 100 mm, for example, of the motor housing 1. Because of this problem the conventional disc grinder 7 has the following disadvantages:
(1) The overall tool configuration is very large and long, thus rendering an extremely poor serviceability especially in a narrow place.
(2) Large-capacity radiator fins 13 and a large-size housing 14 for covering the fins 13 must be employed to cope with increased heat generation while overload is present. Thus, the tool is awkward to operate because of the increased tool weight, giving undue fatigue to the operator.
(3) The tool uses extremely expensive, large-capacity radiator fins 13 and a large fin housing 14 for which the mold is also expensive. Thus, the overall production cost of such tools are remarkably increased.
(4) A large amount of heat will be produced when a tool is operated under an overload. For this particular reason, it is necessary to cool the motor with a large amount of air. To prevent a local heat generation, the prior art has proposed the use of radiator fins provided at the cooling air intake. The radiator fins have a number of folds and a large air-frictional resistance, adversely leading to reduced amount of air and subsequently possible motor burning during overload is being applied.